Rotary transformers are used to transmit data from a rotating object, such as a shaft for example, to a stationary object, such as a control assembly located in a control cabinet.
In the case of standard commercial rotary transformers the data is transmitted by means of slip rings for example.
Rotary transformers are also known, in which data is transmitted from the rotating to the stationary part by means of optical signal transmission. A transmitter disposed in the axial rotation center of the shaft transmits a light signal to a stationary receiver that is likewise disposed in an axially central manner. However such an optical rotary transformer with a standard commercial configuration is not suitable for transmitting signals if the shaft is configured as a hollow shaft, since the transmitter cannot be positioned in the axial rotation center of the shaft but must be positioned on the hollow shaft away from the rotation center, so that data can only be transmitted when the shaft and therefore the transmitter reach a specific angular position, so that transmission to the receiver, which is likewise disposed away from the rotation center, is possible. When using an individual transmitter and an individual receiver it is then generally only possible to transmit data just once per rotation of the shaft, the shaft reaching a specific angular position, specifically when the transmitter and receiver are axially opposite one another, so that the light beam emitted by the transmitter can be received by the receiver.
In the PCT application reference PCT/EP2007/061183 a rotary transformer is described, in which electrical signals are converted to optical signals and transmitted by way of a light-conducting hollow body, which is present in the form of a tube. This allows a rotation gap to be bridged optically. Using light-conducting tubes allows passage of a shaft in the central rotation axis of the rotary transformer, so that the rotary transformer can be positioned in any position on the shaft, since the shaft can be guided through the rotary transformer. So that a receiver at the end of the tube can receive the light signals reliably at any rotation angle, the light must be distributed regularly over the end face of the tube. The tube must therefore have a specific minimum length, as the light cones propagated as a function of the emission angle of the light coupling points should fill all the light-free voids as far as possible. The larger the diameter of the tube, the longer it must be for the light cones leaving the light coupling points to touch one another or to overlay one another to the greatest possible degree. A large number of light coupling points are therefore required to ensure an acceptable tube length even in the case of large tube diameters (larger than approx. 400 mm).
Also the manufacture of a light-conducting tube with a large diameter is very complex. The tubes are generally made of a light-permeable plastic (PMMA) or glass. Therefore to manufacture light-conducting tubes of large diameter (larger than approx. 400 mm), expensive injection molding materials have to be produced. It is also extremely difficult from a manufacturing point of view to produce the necessary thin tubes with a large diameter in this manner. To keep coupling and decoupling losses low the wall thickness of the tubes must be tailored to the diameter of the transmit and/or receiver coupling points. However these are generally only a few millimeters. This is not technically feasible for large-diameter tubes and high coupling and decoupling losses therefore result. The use of light-conducting tubes for optical signal transmission for rotating shafts exceeding a diameter of 400 mm is therefore only technically possible with very high manufacturing outlay and is associated with high coupling and decoupling losses. A large-diameter shaft here is understood to be a shaft with a diameter larger than 400 mm.
A rotary transformer is known from GB 1587531, in which light-conducting fibers are disposed to form a hollow body. Such a procedure is however not technically feasible in practice for large-diameter shafts, for example because of the large number of fibers and the optical connection technology required for the purpose.